This application is based on and incorporates herein by reference Japanese Patent Applications No. 2002-372140 filed on Dec. 24, 2002 and No. 2003-78725 filed on Mar. 20, 2003.
1. Field of the Invention
The present invention relates to a secondary air supply abnormality detection system of an internal combustion engine for detecting an abnormality in secondary air supply, which is performed in order to activate a catalyst disposed in an exhaust passage of the internal combustion engine.
2. Description of Related Art
Conventionally, a method of sensing a flow rate of secondary air directly by using a pressure sensor or a flow rate sensor in order to detect an abnormality in secondary air supply in an internal combustion engine is known. However, in this method, an increase in the cost is unavoidable because of the additional sensors and the like.
A technology relating to the above method is disclosed in Japanese Patent Unexamined Publication No. H06-146867 (pages 2 and 3). In this technology, an oxygen sensor for sensing oxygen concentration in exhaust gas is disposed downstream from an opening of a secondary air supply passage. More specifically, the oxygen sensor is disposed in a secondary air downstream flow, or a flow of the exhaust gas supplied with the secondary air. When an output of the oxygen sensor is reversed, it can be determined that an air-fuel ratio of the secondary air downstream flow is a theoretical air-fuel ratio. Therefore, a method for determining a flow rate of the secondary air (secondary air supply quantity), which is supplied from a secondary air supply system to the exhaust passage, based on an air intake quantity and an operating state of the engine at the time when the output of the oxygen sensor is reversed is proposed.
However, in the above technology, the secondary air flow rate can be calculated only in the case where the air-fuel ratio of the secondary air downstream flow is controlled near the theoretical air-fuel ratio and the output of the oxygen sensor changes rapidly. In addition, there is a possibility that a large calculation error is generated because the secondary air flow rate is calculated in one time.
Other technologies relating to the secondary air supply abnormality detection system for the internal combustion engine are disclosed in Japanese Patent Unexamined Publication No. H05-171973 and Japanese Patent Gazette No. 2576487, for instance.
Japanese Patent Application Unexamined Publication No. H05-171973 (page 2) discloses a technology for heating the catalyst quickly without using a high-capacity air pump. The catalyst can be heated and activated quickly by making the air-fuel ratio of air-fuel mixture supplied to the engine richer and by supplying the secondary air.
A technology disclosed in Japanese Patent Gazette No. 2576487 (pages 1 and 2) prohibits operation for increasing a fuel supply quantity if an abnormality in the secondary air supply system is detected while the fuel supply quantity increasing operation is performed in a heavy load operation. Thus, thermal degradation of the catalyst caused when the catalyst is heated excessively can be prevented. When the abnormality in the secondary air supply system is eliminated, the catalyst can exert normal cleaning effect immediately. Thus, the degradation in the emission can be prevented. The operation range of the internal combustion engine that requires the fuel supply quantity increasing correction is not limited to the heavy load operation range. Other than that, the fuel supply quantity increasing correction is required during warm-up operation after the start or during a transitional period where acceleration and deceleration are repeated, for instance. Moreover, the fuel supply quantity increasing correction corresponding to feedback correction for providing a lean air-fuel ratio is required. If the secondary air supply system is brought to an abnormal state and keeps supplying the secondary air, the temperature of the catalyst increases rapidly and the catalyst main body will be heated excessively. As a result, the temperature of the catalyst will exceed limit temperature (a criterion), above which the thermal degradation of the catalyst is caused.
It is therefore an object of the present invention to provide a secondary air supply abnormality detection system for an internal combustion engine capable of accurately calculating a flow rate of secondary air independently of an air-fuel ratio of a flow downstream of the secondary air, without requiring additional sensors. Thus, the secondary air supply abnormality detection system capable of accurately detecting an abnormality in a system including a secondary air supply system can be provided.
It is another object of the present invention to provide a secondary air supply abnormality detection system for an internal combustion engine enabling a vehicle to travel to a safe place when an abnormality in a flow rate of secondary air supplied from a secondary air supply system is detected, while preventing thermal degradation of a catalyst.
According to an aspect of the present invention, a secondary air supply abnormality detection system for an internal combustion engine includes air-fuel ratio estimating means, air-fuel ratio sensing means, secondary air flow rate calculating means and abnormality determining means. The air-fuel ratio estimating means estimates an air-fuel ratio of air-fuel mixture supplied to the engine. The air-fuel ratio sensing means senses an air-fuel ratio. The secondary air flow rate calculating means calculates a flow rate of the secondary air supplied from the secondary air supply system based on the air-fuel ratio of the air-fuel mixture supplied to the engine, which is estimated by the air-fuel ratio estimating means, and the air-fuel ratio, which is sensed by the air-fuel ratio sensing means when the secondary air is supplied from the secondary air supply system. The abnormality determining means determines an abnormality in a system including the secondary air supply system based on the calculated secondary air flow rate. Thus, the secondary air flow rate can be calculated highly accurately independently of the air-fuel ratio of a flow downstream of the secondary air. As a result, the abnormality in the system including the secondary air supply system can be detected accurately.
According to another aspect of the present invention, the secondary air supply abnormality detection system for the internal combustion engine includes abnormality determining means. The abnormality determining means determines that an abnormality is generated in the system including the secondary air supply system if an air-fuel ratio deviation between the air-fuel ratio of the air-fuel mixture supplied to the engine and the air-fuel ratio, which is sensed by the air-fuel ratio sensing means when the secondary air is supplied from the secondary air supply system, is not in a predetermined range. Thus, the air-fuel ratio deviation can be calculated highly accurately independently of the air-fuel ratio of the flow downstream of the secondary air, and the abnormality in the system including the secondary air supply system can be detected highly accurately.
According to yet another aspect of the present invention, abnormality determining means determines the abnormality in the flow rate of the secondary air supplied from the secondary air supply system. If it is determined that the secondary air supply system is in the abnormal state, air intake quantity supplied to the internal combustion engine is restricted. Thus, temperature increase of a catalyst beyond limit temperature, above which thermal degradation of the catalyst is caused, can be prevented when the secondary air supply system is in the abnormal state. The excessive temperature increase of the catalyst is cause when intake air quantity is excessive.